1. Field of the Invention
The present invention relates to an active noise control apparatus and an active noise control system using the active noise control apparatus which reduces noise by actively controlling vibration, for achieving a beneficial effect of sound insulation.
2. Description of the Related Art
Conventionally, Japanese Laid-Open Patent Publication No. 5-86658 (Patent Document 1) discloses a technique for satisfying a noise insulation performance over a wide frequency range. FIG. 25 illustrates the technique described in Patent Document 1. In FIG. 25, a composite sound insulator 51 includes a surface plate 52 and a damping material 53 having a large loss coefficient (0.2 and above) which is stacked on the rear surface of the surface plate 52. A sound insulation wall is formed by placing the composite sound insulator 51 on a wall surface.
In addition, Japanese Laid-Open Patent Publication No. 6-149271 (Patent Document 2) discloses a conventional technique for actively controlling noise. FIG. 26 illustrates the technique described in Patent Document 2. In FIG. 26, a conversion circuit 65 converts electrical signals obtained by a plurality of vibration sensors 63 into acoustic power which is radiated from a sound insulation wall 61. A control circuit 66 generates a control signal for reducing an output signal of the conversion circuit 65, from an output of a noise sensor 64 and a conversion output of the conversion circuit 65, and outputs the generated control signal to an actuator 62. With this configuration, the actuator 62 suppresses vibration detected by the vibration sensor 63, so that noise can be reduced.
Further, Japanese Laid-Open Patent Publication No. 6-12081 (Patent Document 3) discloses a conventional technique for actively controlling vibration of a panel by using a piezoelectric material as a vibrator. FIG. 27 illustrates the technique described in Patent Document 3. In FIG. 27, vibration sensors 75 and 76 respectively detect vibration excited on first and second panels 71 and 72 by incoming noise from a noise source n. A controller 74 outputs, based on signals inputted from the first and second vibration sensors 75 and 76, a signal for suppressing the vibration of the second panel 72, and applies the suppressing signal to a piezoelectric element 73 via the first and second panels 71 and 72. With this configuration, since the piezoelectric element 73 vibrates in proportion to an applied voltage to suppress the vibration of the second panel 72, noise can be reduced.
Further, Japanese Laid-Open Patent Publication No. 2006-215993 (Patent Document 4) discloses a conventional technique for actively controlling vibration of a panel by using a piezoelectric material as a vibrator. FIG. 28 illustrates the technique described in Patent Document 4. In FIG. 28, a detector 81 detects acceleration generated at a panel 84. A control section 83 generates, based on a signal detected by the detector 81, a control signal for suppressing a noise signal from the panel 84, and outputs the control signal to a vibrator 82. The vibrator 82 attached to the panel 84 transmits its distortion to the panel 84 in accordance with the control signal from the control section 83. With this configuration, since the detector 81 is placed on the panel 84 in a region where the direction of an initial displacement when the control signal is inputted to the vibrator 82 coincides with the direction of a stationary displacement, a control system can be stabilized even when a high gain feedback control is applied, and a vibration suppression effect can be obtained over a wide frequency range.
However, the technique described in the above-mentioned Patent Document 1 has a problem that it is necessary to ensure a large loss coefficient by using a heavy material as a damping material in order to achieve an excellent sound insulation property against noise over a wide frequency range.
In addition, in the technique described in the above-mentioned Patent Document 2, since point vibration is performed by the actuator 62, when a vibration frequency becomes higher, a region in which vibration can be suppressed becomes limited to be just below and near the actuator 62. Accordingly, numerous vibration sensors and actuators become necessary in order to suppress vibration against noise over a wide frequency range. This causes a problem that the size of a control circuit becomes larger.
Further, the techniques described in the above-mentioned Patent Documents 3 and 4 have a problem that since the vibration of the panel, on which the detector is disposed, is reduced by the vibrator, it is necessary to adjust the size of the panel depending on a frequency of noise to be controlled.